Fabrication and Characterization of ZnS/Diamond-Like Carbon Core-Shell Nanowires

We fabricated ZnS/diamond-like carbon (DLC) core-shell heterostructure nanowire using a simple two-step process: the vaporliquid-solid method combined with radio frequency plasma enhanced chemical vapor deposition (rf PECVD). As a core nanowire, ZnS nanowires with face-centered cubic structure were synthesized with a sputtered Au thin film, which exhibit a length and a diameter of ∼10μm and ∼30–120 nm . After rf PECVD for DLC coating, The length and width of the dense ZnS/DLC core-shell nanowires were a range of ∼10μm and 50–150 nm , respectively. In addition, ZnS/DLC core-shell nanowires were characterized with scanning transmission electron microscopy. From the results, the products have flat and uniform DLC coating layer on ZnS nanowire in spite of high residual stress induced by the high sp fraction. To further understanding of the DLC coating layer, Raman spectroscopywas employedwith ZnS/DLC core-shell nanowires, which reveals twoRaman bands at 1550 cm (G peak) and 1330 cm (D peak). Finally, we investigated the optical properties from ultraviolet to infrared wavelength region using ultravioletvisible (UV-Vis) and Fourier transform infrared (FT-IR) spectrometry. Related to optical properties, ZnS/DLC core-shell nanowires exhibit relatively lower absorbance and higher IR transmittance than that of ZnS nanowires.

The stability of nanowires is a key factor that corresponds to their practical applications used on multiassembled nanoscale electrodevices [19,20].In order to fully exploit ZnS nanowires as nanodevices, waveguides, and so forth, it is very important to passivate the surface of the ZnS nanowires, which have structures that exhibit high-chemical reactivity with respect to the deformation, oxidation, corrosion, and contamination of these structures.In this regard, several groups have investigated the protection of ZnS nanowires with boron nitride (BN) or silicon dioxide (SiO 2 ) [21,22].However, core-shell heterostructured ZnS nanowires used in various encapsulating nanomaterials are not yet understood in detail.
In this study, we aimed to introduce ZnS and diamondlike carbon (DLC) core-shell heterostructure nanowires to gain better insight into their applications towards nanoscale electrodevices.DLC is well-known as sp 2 and sp 3 hybridized amorphous carbon materials and can be used in various applications due to the superior mechanical properties with a low friction coefficient, wear resistance, chemical inertness, and biocompatibility [23][24][25].We fabricated ZnS/DLC coreshell heterostructure nanowires by a simple two-step process: the vapor-liquid-solid (VLS) method combined with radio frequency plasma enhanced chemical vapor deposition (rf PECVD).This fabrication is expected to open new possibilities for realizing of a lot of semiconductor nanostructures with DLC coating.

Experimental Methods
A core nanowire, ZnS nanowire, was synthesized by a metalorganic chemical vapor deposition (MOCVD) process using a Zn(S 2 CNEt 2 ) 2 single molecular precursor in a horizontal furnace (Nextron, LABSYS CGF-5000) with a 30 mm diameter quartz tube [26].Zn(S 2 CNEt 2 ) 2 powder (Sigma Aldrich) was then placed upstream of a quartz tube and Au sputtered thin film (thickness ∼ 30 nm) coated Si substrate was positioned downstream of the quartz tube that was initially evacuated till ∼5 mTorr.Subsequently, Ar flow in the quartz tube was set to 100 sccm (SCCM denotes cubic centimeter per minute at standard temperature and pressure (STP)), which enabled the quartz tube to achieve a constant pressure of 300 Torr.After the furnace was stabilized at the process pressure of 300 Torr, the precursor was slowly pushed into the high temperature zone of the furnace and the growth of the ZnS nanowires proceeded for 10 minutes.After the processing of the synthesis, the furnace was cooled down to room temperature.
The as-synthesized ZnS nanowires were moved to the rf PECVD system with 13.56 MHz.As for PECVD process, vacuum chamber was evacuated to 4 × 10 −5 Torr for ultraclean and stable plasma generation.Methane (CH 4 ) with a flow of 10 sccm was then put into the chamber, which was allowed to reach 3 × 10 −2 Torr for the DLC deposition conditions.To facilitate the deposition of a thin DLC coating on the ZnS nanowires, rf power was applied at 100 W for 60 seconds.Different from the as-synthesized ZnS nanowires in gray, the ZnS/DLC core-shell nanowires were a light-yellow color.

Results and Discussion
Figure 1 shows a representative XRD pattern of ZnS/DLC core-shell nanowires.All the diffraction peaks were identified as a face-centered cubic (FCC) phase of ZnS (JCPDS: 05-0566), while XRD peaks of DLC were hardly detected due to its amorphous structure.A morphological study of the ZnS/DLC core-shell nanowires was performed using SEM equipped with a dual beam focused ion beam system.Figure 2(a) displays a low-magnification SEM image of the ZnS/DLC core-shell nanowires.The length and width of the dense ZnS/DLC core-shell nanowires on Si substrate were in a range of ∼10 m and 50-150 nm, respectively.The inset of Figure 2(a) shows SEM image of the as-synthesized ZnS nanowires with a length and a width of ∼10 m and ∼30-120 nm, respectively.Figure 2 A detailed structural and chemical analysis of ZnS/DLC core-shell nanowires was conducted by TEM.For the sample preparation of TEM analysis, ZnS/DLC nanowires were directly dispersed onto a SiO 2 thin film on a TEM grid. the DLC layer is very flat and uniform without any pretreatment performed on the ZnS nanowires.DLC is generally known to be a material that is difficult to deposit on any substrate since it has high residual stress induced by the high sp 3 fraction [27].Therefore, pretreatments, for instance, ion bombardment on an arbitrary substrate, have been applied to the DLC deposition to promote good adhesion between the film and the substrate.
To further investigate the chemical information of the ZnS/DLC core-shell nanowire, we conducted a scanning TEM (STEM) annular dark field (ADF) analysis with EDS line scan profile.Figure 3(c) presents ADF image and the EDS line-profile analysis crossed with the growth direction laid on the ADF image of the ZnS/DLC core-shell nanowire.This evidently shows that only zinc (red) and sulfur (blue) were present within the ZnS nanowire, which has an intrinsic chemical composition without interdiffusion at the interface between ZnS and DLC.Carbon (green), on the other hand, was uniformly distributed along the width direction of the core-shell nanowires.Figure 3(d) illustrates EDS spectrum obtained from the core-shell, which indicates that Cu, Si, and O have originated from the TEM grid.
In order to gain further understanding of the DLC bonding state, Raman spectroscopy was employed.Figure 4(a) displays the Raman spectrum of the ZnS/DLC core-shell nanowires, which exhibits two kinds of Raman peaks that were observed at 1550 cm −1 (G peak) due to E 2g symmetry and vibration of sp 2 bonding in graphite carbon, and at 1330 cm −1 (D peak) regarding the breathing modes of sp 2 bonding in disordered graphitic carbon.Among many kinds of DLC materials, DLC deposited on ZnS nanowires are the most likely typical hydrogenated diamond-like amorphous carbon, which is in good agreement with the previous results of DLC deposited with PECVD [28].
To explore the optical properties of the ZnS/DLC coreshell nanowires, we conducted UV-Vis and FT-IR spectrometry from ultraviolet to infrared wavelength region.As for the measurement of the optical spectrum, we directly dispersed ZnS/DLC core-shell and ZnS nanowires onto a glass substrate and the measurements were made at least 20 times on each sample to ensure reliability.Firstly, we investigated absorption properties ZnS/DLC core-shell and ZnS nanowires between ultraviolet and infrared wavelength regime using UV-Vis spectrometry.As shown in Figure 4(b), absorbance of ZnS/DLC nanowires is relatively lower than that of ZnS nanowires overall measured wavelength, which might be due to reflectance reduction from DLC coating (DLC refractive index: 1.8, ZnS refractive index 2.3).Further, we observed absorption peaks 313 nm and 326 nm of ZnS/DLC core-shell and ZnS nanowires, respectively.Absorption peak of bulk ZnS is generally known as 335 nm.In this regard, UV-Vis analysis indicates blue shifted spectrum of 9∼22 nm both ZnS/DLC core-shell and ZnS nanowires, which is conceivably attributed to quantum confinement effect [29,30].Subsequently, we performed FT-IR analysis in order to gain IR transmittance.The FT-IR spectrum at a wavenumber range of 1400 cm −1 to 4000 cm −1 , as shown in Figure 4(c), reveals that the ZnS/DLC core-shell nanowires (black) of IR transmittance were enhanced with values of ∼1.2% higher than those of the ZnS nanowires.In general, ZnS and DLC combined structure is very well known for enhanced transmittance system in visible and infrared wavelength period such as solar cell, military device [31].More generally, DLC provides lower refractive index surface (∼ 1.8) to inner ZnS nanowire.However, in our ZnS/DLC coreshell and ZnS nanowires, difference of IR transmittance is about 1.2%.Even though IR transmittance value between ZnS/DLC and ZnS nanowires seems to be small, we guess that this enhanced IR transmittance is due to DLC coating layer.Further, we suggest that difference of IR transmittance is only due to single surface reflection of nanoscale compared to bulk ZnS/DLC system including single and multiple reflection within visible and infrared region.The fabrication process developed in this work is a very simple two-step process consisting of Au-catalyzed VLS growth and sequential rf PECVD and is able to synthesize ZnS/DLC.There is still a limitation to obtaining fully uniform core-shell heterostructured nanowires due to the randomly oriented nature of the initial nanowires on the substrate.However, our fabrication technique may open up the superior stack coverage on 1D nanostructures using DLC as an outer layer deposited by common PECVD.Moreover, this new process may be more feasible and effective platform for multiassembled nanodevices with elaborate deposition technology.

Conclusions
In summary, ZnS/diamond-like carbon core-shell heterostructure nanowires were successfully synthesized by VLS growth of ZnS nanowires and a sequential rf PECVD process.ZnS nanowires were initially synthesized with a width of 70-80 nm and DLC layers were continuously deposited on the surface of ZnS nanowires with a thickness of 20-30 nm.The detailed analysis of the carbon structure of the DLC with Raman spectroscopy confirmed that two kinds of Raman bands observed at 1550 cm −1 (G peak) and at 1330 cm −1 (D peak) indicated DLC to be hydrogenated diamond-like amorphous carbon.Through UV-Vis spectrometry for optical properties investigation, absorbance of ZnS/DLC core-shell nanowires seems relatively lower than that of ZnS nanowires with blue-shift of absorption peak within wavelength of 300∼500 nm.As for FT-IR spectrometry analysis between 1400 cm −1 and 4000 cm −1 wavelength at infrared regime, we observed enhanced IR transmittance of ZnS/DLC coreshell nanowires with a value of about 1.2% compared to ZnS nanowires.The heterostructure core-shell nanowire developed in the present work can be applied to a wide range of nanoscale electrodevices, which not only introduces novel perspectives for applying DLC on a semiconductor surface but also establishes a nanoscale fabrication technique for future nanodevices.
(b) shows a high magnification SEM image of ZnS/DLC coreshell nanowires.Using the transmission mode of SEM, the outer layer of the DLC on the ZnS core nanowires can be clearly seen to have a thickness of ∼20 nm.The inset of Figure 2(b) shows an enlarged SEM image of the as-synthesized ZnS nanowires.As shown in the inset of Figure 2(b), Au catalyst is clearly observed at the tip of an as-synthesized ZnS nanowire, indicating that the growth of ZnS nanowires was caused by the VLS method.